The present invention relates, in general, to securing semiconductor chips to leadframes, and more particularly, to methods for nonadhesively securing semiconductor chips to leadframes.
An integral step in packaging a semiconductor chip is securing or attaching the semiconductor chip to a leadframe. Typically, the back side of the semiconductor chip is bonded to the leadframe or a heatsink using an adhesive agent such as an epoxy die attach material or a lead based solder. Subsequently, the die attach material is cured, and the semiconductor chip is encapsulated by a molding compound to form a packaged semiconductor chip.
Although this technique is widely used for encapsulating semiconductor chips, it suffers from several drawbacks. For example, during the step of curing the epoxy die attach material an organic solvent is released or outgassed from the epoxy die attach material. The outgassed organic solvent forms a layer of organic contaminants on the semiconductor chip and leadframe that weakens the bond between the semiconductor chip and the leadframe. The weakened bond may result in delamination of the molding compound from the semiconductor chip, the leadframe, or both. Delamination often leads to solder reflow cracking of a plastic package when moisture is present on the semiconductor chip and the plastic package is subjected to thermal shock such as occurs when soldering a surface mount device to a printed circuit board. Solder fellow cracking is also referred to as "popcorn" cracking. Further, semiconductor chip tilting may occur with the use of adhesive die attach materials. Lead based solders, on the other hand, pose an environmental hazard.
Accordingly, it would be advantageous to have a method for securing a semiconductor chip to a leadframe that eliminates the use of adhesive materials. Moreover, the method should secure the semiconductor chip to the leadframe in such a manner as to preclude the occurrence of semiconductor chip tilting. It would be of further advantage for the method to be compatible with environmentally safe manufacturing techniques.